Maternal Effect Genes: Transcription that terminates during meiotic maturation of mouse eggs, resumes only after robust activation of the embryonic genome of the two-cell embryo. This interregnum between oocyte and embryonic gene transcription dictates a role for stored maternal factors. Encoded by maternal effect genes, these components accumulate during oogenesis and provide pathways and organelles essential for fertilization, activation of the embryonic genome and cleavage stage embryogenesis. Using mouse transgenesis, the role of individual or complexes of maternal factors is investigated in these processes. Fertilization: The taxon-specificity of sperm-egg recognition in mammals that results in fertilization is mediated primarily by the zona pellucida, an extracellular matrix surrounding ovulated eggs. Although a simple structure of 3-4 glycoproteins, the molecular basis of this sperm binding to the zona pellucida has been controversial. Current studies investigate: 1) molecular requirements to support taxon-specific sperm-egg recognition on the surface of the zona pellucida; 2) mechanism that result in acrosome exocytosis necessary for gamete fusion; and 3) the processes by which post-fertilization polyspermy is prevented. Early Development: Activation of the embryonic genome in mice begins late in the one-cell zygote and is fully underway by the two-cell cleavage stage. Initial cell lineages that presage the inner cell mass and extra-embryonic trophectoderm are established when eight blastomeres compact to form polarized morulae in pre-implantation mouse development. The role of maternal effect genes and organelles in these processes in mammals constitute a rapidly evolving area of inquiry. Current studies investigate: 1) novel maternal effect genes that affect early mouse development; 2) the role of maternal organelles in the maternal to zygotic transition; and 3) the role of degradation of maternal components in activation of the embryonic genome.